1. Field of the Invention
This invention relates to a reel pipelaying system and vessel on which at least one reel is disposed for laying a pipeline in waters having depths limited only by the strength of the pipe. More particularly, the invention pertains to a new type of pipeline layout system and vessel in which at least one reel is employed for storing and unspooling a rigid walled pipeline. The laying system provides for the layout of a pipeline from any one of a plurality of reels and for the adjustment of water entry angles over a wide angular range.
The vessel of this invention is designed to accommodate a permanently mounted pipe spooling main reel which is of substantial size and is capable of spooling pipe up to 16 inches in diameter. The laying system is stern mounted for handling the layout during unspooling from this permanently mounted main reel or from auxiliary storage reels.
2. History of the Prior Art
In laying offshore subsea pipelines for such uses as the gathering of oil and/or gas from offshore wells, as, for example, in the Gulf of Mexico, it has been conventional to use one of two main methods to lay the pipe. In the first, called the "stovepiping" method, a pipeline is fabricated on the deck of a lay barge by welding together individual lengths of pipe as the pipe is laid out from the barge Each length of pipe is about 40' or 80' long. Thus, the pay-out operation must be interrupted periodically to permit new lengths of pipe to be welded to the string. The stovepiping method requires that skilled welders and their relatively bulky equipment accompany the pipelaying barge crew during the entire layout operation; all welding must be carried out on site and often under adverse weather conditions. Further, the stovepiping method is relatively slow, with experienced crews being able to lay only one to two miles of pipe a day. This makes the entire operation subject to weather conditions which can cause substantial delays and make working conditions quite harsh.
The other principal conventional method is the reel pipelaying technique. In this method, a pipeline is wound on the hub of a reel mounted on the deck of a lay barge. Pipe is generally spooled onto the reel at a shore base. At such a shore base, short lengths of pipe can be welded under protected and controlled conditions to form a continuous pipeline which is spooled onto the reel. The lay barge is then towed to an offshore pipelaying location and the pipeline is spooled off the reel between completion points. This method has a number of advantages over the stovepiping method, among them, speed (one to two miles per hour advantage); lower operating costs, e.g., smaller welding crews and less welding equipment must be carried on the lay barge; and less weather dependency.
The broad concept of reel pipelaying was also disclosed in British Patent No. 601,103 (Ellis) issued Apr. 28, 1948, wherein it was suggested that lengths of pipe be joined together at the manufacturing plant and coiled onto a drum and mounted on a barge or ship; the loaded barge would then be moved to the desired marine location and the pipe unwound from the drum by fixing one end of the pipe and towing the barge away from the fixed location.
After a hiatus of about thirteen years, research into the reel pipelaying technique was renewed and was carried on by Gurtler, Herbert & Co., Inc. of New Orleans, La. By 1961 that company had sufficiently advanced the reel pipelaying technique to make it a commercially acceptable and viable method of laying pipe in the offshore petroleum industry, able to compete with the traditional stovepiping technique. The first known commercial pipelaying reel barge, called the U-303, was built by Aquatic Contractors and Engineers, Inc., a subsidiary of Gurtler, Herbert, in 1961. The U-303 utilized a large vertical axis reel, permanently mounted on a barge and having horizontally oriented flanges (generally referred to in the trade as a "horizontal reel"). A combined straightener-level winder was employed for spooling pipe onto the reel and for straightening pipe as it was unspooled. The U-303 first laid pipe commercially in September 1961 in the Gulf of Mexico off the coast of Louisiana and was used successfully during the 1960's to lay several million linear feet of pipe of up to 6" diameter. The U- 303 reel pipelaying barge is described in U.S. Pat. No. 3,237,438 (Tesson) and U.S. Pat. No. 3,372,461 (Tesson) both assigned to the assignee of the invention hereof.
The successor to the U-303 currently in use in the Gulf of Mexico and known in the trade as the "Chickasaw" also utilizes a large horizontal reel, permanently mounted to the barge such that it is not readily moveable from one carrier vessel to another. Various aspects of "Chickasaw" are designed in the following U.S. Patents, all assigned to the assignee of the invention hereof:
Sugasti, et al.--U.S. Pat. No. 3,630,461 PA1 Gibson--U.S. Pat. No. 3,651,778 PA1 Mott, et al.--U.S. Pat. No. 3,680,342 PA1 Key, et al.--U.S. Pat. No. 3,712,100. PA1 Springett, et al--U.S. Pat. No. 4,230,421 PA1 Uyeda, et al--U.S. Pat. No. 4,269,540 PA1 Yenzer, et al--U.S. Pat. No. 4,297,054 PA1 Springett, et al--U.S. Pat. No. 4,340,322 PA1 Uyeda, et al--U.S. Pat. No. 4,345,855
The Gibson patent shows an apparatus for diverting a single pipeline from a horizontal unspooling direction to a vertical direction for layout in a body of water. The patent describes very simple unspooling tools which do not provide for certain important pipeline handling functions.
U.S. Pat. No. 3,685,306 to Mott also describes an apparatus which diverts a single pipeline from a horizontal position to a fixed vertical direction. The pipeline can be successively unreeled from adjacent ganged reels but the water entry angle cannot be adjusted to permit layout in variable water depths.
Commercial reel pipelaying techniques require the use of certain pipe handling equipment in addition to the reel. Among such pipe handling equipment usually employed in commercial reel pipelaying systems is a straightener mechanism. This may take the form of a series of rollers or tracks, or other arrangement which imparts sufficient reverse bending force to the pipe to remove residual curvature so that after unspooling, the pipe will lay substantially straight on the sea bottom.
Other pipe handling equipment used in commercial reel pipelaying systems must be provided for facilitating pipeline adjustments, particularly for inspecting pipeline coatings and repairing the same. Valves must be inserted into the pipeline at various lengths and for this operation pipe clamping and holding equipment is provided. Various sacrificial anodes, survey markers, transponders and floats must also be attached to the pipeline in a pipe handling station for many projects. Commercial reel pipelaying systems also provide equipment for making connections with and to existing pipeline structures. For making such connections pipe clamping means are required.
U.S. Pat. No. Re. 30,846 (Lang et al) describes an apparatus for laying pipeline from a vertical reel in which the pipe conditioning apparatus is pivotable to adjust the lift-off angle of the pipe relative to the horizontal plane (e.g., the deck of a ship) as a function of the water depth in which the pipe is being laid. This has distinct commercial advantages, especially where the reel pipelaying system is incorporated into a self-propelled ship, such as that of the present invention, capable of traveling to different job sites, having different pipe size and/or laying depth requirements.
An early concept for a reel pipelaying ship is described in Goren, et al. "The Reel Pipelay Ship--A New Concept" Offshore Technology Conference Proceedings, May 1975 (Paper No.--OTC 2400) This paper (hereafter the Goren, et al 1975 OTC Paper) describes advantages and operating features of a proposed reel pipelaying ship. However, the cost of construction of a ship as described in that paper was estimated to be on the order of $100,000,000.